The present invention relates to the art of electromagnetic field correction and modification. It finds particular application in conjunction with establishing temporal dependence of magnetic field gradients in magnetic resonance imaging and will be described with particular reference thereto. It is to be appreciated, however, that the invention will also find application in conjunction with magnetic resonance spectroscopy and other applications in which eddy currents degrade electromagnetic field temporal dependence.
In magnetic resonance imaging and spectroscopy, a uniform magnetic field is created through an examination region in which a subject to be examined is disposed. The magnetization vector of dipoles in the examined subject preferentially aligns with the uniform field. Radio frequency excitation pulses are supplied to cause the magnetization vectors to precess about the uniform field. After the radio frequency excitation, the precessing magnetization vectors generate radio frequency magnetic resonance signals as the precession decays back toward alignment with the uniform magnetic field. The frequency of the radio frequency resonance signals is proportional to the strength of the magnetic field. Various combinations of radio frequency pulses and magnetic field gradient pulses are applied to manipulate the precessing magnetization vector to create magnetic resonance signals, such as echo signals.
In magnetic resonance imaging, gradient magnetic fields are applied to select and encode the magnetic resonance signals. The magnetic field gradients are applied to select one or more slices or planes to be imaged. Further gradient fields are applied for selectively modifying the uniform magnetic field to encode frequency and phase into the magnetization vectors, hence, the resonance signals, which identify spatial location within the selected plane.
The gradient fields are conventionally applied as a series of gradient pulses. Specifically, electrical current pulses are applied to gradient field magnets adjacent the image region. A profile, or particular temporal dependence, is selected for the current pulse in accordance with the profile of the gradient magnetic field to be applied, commonly a square wave or step function, a trapezoid, or other ideal wave shapes.
One of the inherent problems is that the profile of the gradient magnetic field pulse does not match the profile of the electrical current pulse. A changing magnetic field induces eddy currents in adjacent electrically conductive structures. Each eddy current causes a corresponding eddy magnetic field. Thus, the resultant gradient pulse causes eddy currents that add unwanted eddy components to the induced gradient pulse. The effect of the eddy current varies with the amount and conductivity of the material in which the eddy current is induced, the proximity of the material to the gradient coil, and the magnitude of the pulsed gradient magnetic field. The metallic structures might include supporting structures of the magnet, a room temperature bore tube, a liquid nitrogen dewar for superconducting magnets, other gradient field coils, and the like. Construction tolerances cause the eddy current response to vary from unit to unit.
In order to improve image quality, the shape of the electric current pulses is altered such that the magnetic field produced by the sum of the current pulse and the corresponding eddy currents approximates the desired gradient magnetic field profile. Commonly, the current pulse correction circuit includes a plurality of filters whose characteristic frequency is adjustable. An amplifier with an adjustable gain is associated with each filter. Typically, a large array of potentiometers is provided for adjusting the characteristic filter frequencies and the gain corresponding to each. In this manner, selected frequency components of the current pulse are enhanced or suppressed as may be necessary such that the gradient magnetic field produced by the sum of the current pulse and the induced eddy currents has a substantially preselected profile.
Manual calibration, which is very difficult and time consuming, is commonly achieved by trial and error. The time for performing the calibration may vary widely with the skill and luck of the calibrating technician. Although some guidance might be obtained from units of the same model, manufacturing tolerances provide a sufficiently wide variation in resultant eddy currents that a unique calibration is required for every unit.
Another drawback to manually adjusted systems is that the calibration is frequently not optimized. The eddy current compensation components are frequently interrelated. Adjusting the frequency of one of the filters or its corresponding gain commonly alters the response of the other eddy currents as well. Thus, a small change to improve the response at one frequency may cause significant error at others. The calibration process is often terminated when the magnetic field response is brought within predefined specifications, rather than when the calibration is optimized.
The present invention provides a new and improved automatic calibration method and apparatus which overcomes the above referenced drawbacks and others.